Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
Lithium/sulfur (Li/S) batteries are particularly attractive because of the balance between high specific energy (i.e., >350 Wh/kg has been demonstrated), rate capability, and cycle life (>50 cycles). Only lithium/air batteries have a higher theoretical specific energy. Lithium/air batteries, however, have very limited rechargeability and are still considered primary batteries.
A drawback that is common to many lithium ion batteries results from the fact that the chemistries incorporate phase-change materials that exhibit voltage plateaus dependent upon the particular cell chemistry, resulting in a very flat open-circuit potential (OCP) over the normal operating voltage of the cell. Battery state of charge (SOC), however, is typically estimated using a combination of two techniques: coulomb counting and OCP measurement.
Coulomb counting involves integrating the current that is passed to or from the cell to calculate the change in the cell's capacity. Errors in current measurement render this technique inaccurate over time, while side reactions in the cell lead to further deviations between the estimated and actual SOC. By measuring or estimating the OCP, or rest potential, of the cell, one may use OCP-SOC functional relationships to extract the SOC. The coulomb-counting technique tends to be more accurate at short times or when the current is high, while the OCP technique does better when the cell is at rest or the current is low. The two techniques of SOC estimation are typically combined in a number of different ways to obtain the most accurate estimate of SOC possible at all times.
Thus, flat or shallowly sloping OCPs, while providing some advantages, make accurate SOC estimation very difficult. Accordingly, for cells with a flat (or shallowly sloping) OCP, the OCP-SOC correlation technique does not provide the desired accuracy in determination of the cell SOC. Since coulomb counting alone is inherently inaccurate, a need exists for alternative SOC estimation techniques for lithium ion batteries.
What is needed therefore is a battery system and method that provides the advantages of chemistries which exhibit a flat or shallowly sloping OCP while providing a more accurate SOC determination. A system which could also be used to provide an indication of overpressure conditions in a cell would be beneficial.